1. Field of the Invention
The present invention generally relates to an image processor for use in a facsimile, a copying machine or the like, and in particular to image processing apparatus and method for converting a color image into a monochrome image.
2. Description of the Related Art
In the case of transmitting an original color image to a monochromatic facsimile apparatus such as a Group III facsimile, it is necessary to convert the color image into a black-and-white image based on the lightness of color. Such a black-and-white image is further converted into facsimile data to be transmitted through the public telephone network. Therefore, when a color image having colors of similar lightness (for example, red-color characters on a green-color paper of almost the same lightness) is transmitted, a facsimile apparatus at a sending side reads the colors as the same black-and-white image. Thus, the facsimile apparatus at the receiving side cannot discriminate between the characters and the background, resulting in missing information.
As a method for solving the above problem caused by converting a color image into a monochrome image in facsimile transmission, there has been proposed a facsimile machine in Japanese Patent Application Laid-open Publication No. 6-70180, for example. The conventional facsimile machine converts a color image into a black-and-white image having a plurality of levels of density corresponding to different colors of the color image and then transmits it to the opposite party. Therefore, at the receiving side, it is possible to discriminate between different colors even if the colors are of almost the same lightness.
More specifically, according to the conventional facsimile machine, different half-tone monochrome image levels are assigned to the three principle colors of red, green and blue, and the half-tone monochrome image is transferred by facsimile. According to this system, a full color image read by a scanner is converted into a multi-color image of a plurality of colors by a multi-color converter, and then the multi-color image is converted to a black-and-white image with a plurality of monochrome density levels corresponding to different colors of the multi-color image. In this case, the multi-color image is converted into a black-and-white image of a plurality of black-and-white patterns. Such a black-and-white image is transmitted to the receiving side.
However, the above-described conventional apparatus has the following disadvantages. Although it is possible to transfer the black-and-white image discriminating between different colors of the same lightness, a sender finds it difficult to imagine the monochrome image output produced by the conversion.
Further, since the monochromatic image to be transmitted is composed mainly of a half-tone image, the amount of data increases and the time required for facsimile transmission becomes longer. However, in most cases, senders at a transmitting side already know that a receiver uses a facsimile apparatus for producing a monochromatic output. Therefore, the purpose of transmitting a monochrome image produced from a color image is to transmit a layout of the color image or to transmit only graphical information like a map or character information, and therefore, the image does not necessary require to be an half-tone image.
An object of the present invention is to provide image processing apparatus and method that are capable of converting a color image into a monochrome image which can discriminate between different colors with the same lightness and further can be easy for an operator to imagine the converted monochrome image.
Another object of the present invention is to provide an image processing apparatus for use in a facsimile machine, that is capable of converting a color image into a black-and-white image which can discriminate between different colors with the same lightness, further can be easy for a sending party to imagine the converted black-and-white image, and furthermore can improve the transmission efficiency.
According to the present invention, the color image is converted to a monochrome input image and a plurality of primary-color input images and then boundary information is extracted from the plurality of primary-color input images. The boundary information discriminates between different colors with the same lightness in the color image. Thereafter, the boundary information is superimposed as a binary image on the monochrome input image to produce the monochrome image. Since the boundary information is extracted and superimposed on the monochrome input image, a boundary between different colors with the same lightness can be easily imagined. In the case of facsimile transmission, such a boundary can be easily imagined by a sending party and can be visible at the receiving party.
The boundary may be extracted by: detecting an edge of each of the primary-color input images to produce edge detection images of the primary-color input images; and combining the edge detection images to produce a combined edge detection image as the boundary information. In this case, the combined edge detection image is superimposed on the monochrome input image to produce the monochrome image where a boundary discriminates between different colors with the same lightness.
The superimposing may be preferably performed by: detecting an edge of the monochrome input image to produce a monochrome edge detection image; extracting the boundary by exclusively combining the combined edge detection image with the monochrome edge detection image; and removing the boundary from the monochrome input image to produce the monochrome image.
In the case of data transmission, it is preferably that the monochrome input image is converted into a binary input image and then the combined edge detection image is removed from the binary input image to produce the monochrome image. Alternatively, it is further preferably that an edge of the monochrome input image is detected to produce a monochrome edge detection image and the monochrome input image is converted into a binary input image. Thereafter, the boundary is extracted by exclusively combining the combined edge detection image with the monochrome edge detection image, and then the boundary is removed from the binary input image to produce the monochrome image.